1. Field of the Invention
The present invention relates to a cushioning pad for hot press. More particularly, the present invention relates to a cushioning pad for hot press which is used when an object product is subjected to pressing or thermocompression bonding in the process of producing a precision instrument component such as a printed circuit board including a copper-clad laminate, a flexible printed circuit board, and multilayer lamination board, an IC card, a liquid crystal display panel, or a ceramic lamination board (referred to as the laminated plate in the present invention hereinafter). Especially, the present invention relates to a cushioning pad for hot press which can be suitably used for hot press the laminated plate such as the multilayer plate for which a very high thickness precision is required.
2. Description of the Background Art
In the process of pressing or thermocompression bonding in manufacturing the laminated plate, there is used a method of sandwiching a laminated plate material 21 which is an object to be pressed between hot plates 22 and 22 and applying a predetermined pressure and heat thereto as shown in FIG. 4. In order to obtain a product of high precision, it is necessary to apply the heat and the pressure uniformly over a whole surface of the laminated plate material 21 during the hot press. For the purpose of the above, the hot press is performed with a planar cushioning pad 23 interposed between the hot plate 22 and the laminated plate material 21.
Conventionally, as the cushioning pad for hot press 23, various kinds of materials are used, such as kraft paper, organic or inorganic fibers bonded with a binder, rubber, nonwoven fabric, and a laminated body of the rubber and the nonwoven fabric. Materials other than the kraft paper can be repeatedly used in the pressing basically.
Japanese Unexamined Patent Publication No. 55-101224 discloses a cushioning pad for hot press formed of the nonwoven fabric. According to Japanese Unexamined Patent Publication No. 55-101224, a multilayer needle felt cushioning pad is disclosed in which a batt fiber and a foundation cloth are laminated into a plurality of layers and integrally bonded by a needle punch and finished by a heat treatment.
Japanese Utility Model Publication No. 48-31033 discloses a cushioning pad for hot press formed of a laminated body including the nonwoven fabric and the rubber. According to Japanese Utility Model Publication No. 48-31033, cushioning pad for pressing is disclosed in which between two needlepunched bonded fiber layers, an elastic sheet having a thickness smaller than that of the bonded fiber layer is integrally provided without using an adhesive agent.
The cushioning pad for hot press disclosed in either Japanese Unexamined Patent Publication No. 55-101224 or Japanese Utility Model Publication No. 48-31033 comprises the nonwoven fabric. Since the cushioning pad for hot press comprising the nonwoven fabric has a preferable cushion property because it contains airspace inside, it is used in forming a general copper-clad laminate or printed circuit board in many cases.
However, recently, since the laminated plate such as the printed circuit board has been miniaturized and requires high precision, it is necessary to manufacture a multilayer plate in which precisely printed circuits are laminated into a plurality of layers, with high thickness precision. When the laminated plate which requires high thickness precision is hot pressed, the cushioning pad comprising the nonwoven fabric is not suitable because it contains the airspace and its cushion property is too much in the thickness direction.
Japanese Utility Model Publication No. 5-22400 discloses a cushioning pad for hot press. According to Japanese Utility Model Publication No. 5-22400, a cushioning pad for hot press is disclosed in which a silicone rubber layer is formed on each surface of a glass fiber foundation cloth and a glass fiber cloth impregnated with a conductive fluoro resin is integrally provided on each surface thereof.
The cushioning pad for hot press disclosed in the Japanese Utility Model Publication No. 5-22400 comprises two rubber layers and each rubber layer is sandwiched between glass clothes positioned upper and lower surfaces thereof. Since the cushioning pad does not contain airspace inside thereof, thickness precision of an object to be pressed can be easily achieved. However, according to the cushioning pad for hot press disclosed in Japanese Utility Model Publication No. 5-22400, an end of the rubber layer is cut into strips after it is used repeatedly in the hot press.
Problems of the cushioning pad for hot press disclosed in Japanese Utility Model Publication No. 5-22400 are schematically described with reference to the drawings. FIG. 5A shows a cushioning pad for hot press 24 comprising two rubber layers 25 and each rubber layer 25 is sandwiched between inorganic fiber woven fabric layers 26 such as glass cloth positioned on its upper and lower surfaces. FIG. 5B shows a state of the end of the cushioning pad 24 during the hot press. When the cushioning pad 24 is used in the hot press, the rubber layer 25 protrudes in a planar direction when it is pressurized. Meanwhile, since the inorganic fiber woven fabric 26 positioned on the upper and lower surfaces of the rubber layer 25 has a high modulus of elasticity in tension, its dimension is hardly changed in the planar direction, so that the upper and lower surfaces of the rubber layer 25 are fixed by the inorganic fiber woven fabric 26 and cannot extend in the planar direction. Therefore, a middle part of the end of the rubber layer 25 in the thickness direction becomes a protrusion 27. When the pressure is released, the protrusion 27 is returned to its original position by dimension stability of the inorganic fiber woven fabric 26 and elastic restoring force of the rubber layer 25, so that the cushioning pad 24 is returned to the state shown in FIG. 5A. During repeated use of the cushioning pad in the hot press, since the upper and lower surfaces of the rubber layer 25 are fixed by the inorganic fiber woven fabric 26 and cannot extend, only the middle part of the rubber layer 25 in the thickness direction repeats protrusion and restoration. As a result it is thought that a cut strip 25a is generated at the end of the rubber layer 25 in an early stage because of a stress caused by the repeated protrusion and restoration as shown in FIG. 5C.
In order to solve the problem in the cushioning pad for hot press having the structure shown in FIGS. 5A to 5C, the inventors of the present invention made an attempt to replace the inorganic fiber woven fabric with an organic fiber woven fabric having a relatively low modulus of elasticity in tension, that is, a cushioning pad 28 in which upper and lower surfaces of a rubber layer 29 are sandwiched between an organic fiber woven fabric 30 such as a meta aromatic polyamide fiber having a relatively low modulus of elasticity in tension as schematically shown in FIG. 6A. In this case, the organic fiber woven fabric 30 can extend in the planar direction to some extent. Since the organic fiber woven fabric 30 can also extend along with the protrusion of the rubber layer 29 at the time of the press, the stress imposed on the rubber layer 29 is reduced. However, in this case, since the organic fiber woven fabric 30 extends itself when it is pressurized, it is necessary that the whole configuration of the cushioning pad 28 should be restored by the elastic restoring force of the rubber layer 29 only at the time of releasing the pressure. Thus, the cushioning pad 28 shown in FIG. 6 is inferior in configuration restoring property as compared with the cushioning pad 24 shown in FIGS. 5A to 5C. As a result, while the cushioning pad 28 shown in FIG. 6 is repeatedly used in the hot press, the cushioning pad 28 is elongated in an early stage as a whole in the planar direction because of fatigue of the rubber layer 29, so that the thickness precision deteriorates. Further, as the cushioning pad 28 is repeatedly used in the hot press, a damaged part 31 is also generated at the end of the rubber layer 29 as shown in FIG. 6B.